PCB Design Fundamentals: Overview

Overview

The National Instruments PCB Design Fundamentals Series is your free resource on the Internet for learning about prototyping Printed Circuit Boards (PCBs). This selection of resources includes an introduction to terminology, design best-practices and additional such as prototype fabrication.

The series is divided among a number of in-depth detailed articles that offer HOW TO information on the important concepts and details of PCB Design.

Table of Contents

1. Introduction to PCB Design

A Printed Circuit Boards (PCB) is a rugged, copper and non-conductive substrate based structure to connect electrical components (for example the green board inside a common electrical appliance is a PCB). The PCB is the backbone of electrical devices, allowing you to connect passive (resistor, inductor, capacitors etc…), active (operational amplifiers etc…) and embedded devices together, into specific form factors to fit the design need. Connections between the components are made through copper connections (routes) which become passageways for electrical signals.

PCBs were first developed by an Austrian Engineer named Paul Eisler. Born in Vienna, and educated at the Vienna University, he made his way to the United Kingdom in the 1936. He began developing the circuit manufacturing process during the Second World War where he earned a number of patents dealing with the etching process, to define the various routes and electrical conduits on your board.

2. PCB Design Flow

The PCB design flow consists of four distinct stages. There is the part research and selection stage, schematic capture and simulation, board layout, and finally board verification.

Figure 2 - The PCB Design Flow

If you are unfamiliar with any of these stages, I recommend that you view the following page, that will be able to provide you additional details on each stage, and how best to handle design: Best Practices in PCB Design.

The remainder of this page will talk about board layout and design. Prior to this layout stage, you must first define a schematic capture (or circuit diagram) in a tool such as NI Multisim. A schematic capture tool allows you to place symbols for electronic components and wire them together. Each of these symbols (for the amplifier, resistor etc…) are linked to a symbol that represents the dimensions and shape of an actual device used on a PCB. So a resistor symbol is associated with a real-world resistor footprint or landpattern. This landpattern is what we use on our board layout in order to define our final PCB.

Figure 3 - Schematic Capture

You must transfer a schematic to a board layout application (such as NI Ultiboard). From here you define the board outline (form factor), place parts onto the board surface (landpattern placement) and route copper connections (make pathways for signals to be conducted through the PCB).

Figure 4 - Board Layout

After these stages, you can export your design to an industry standard format (Gerber) from which a physical board is fabricated. The rest of this page will speak to this physical hardware, and what it consists of.

3. Understanding the Make-Up of a PCB Design

A PCB typically consists of multiple layers of copper which are used to conduct a signal, with various layers of dielectric for insulation. The green color that one finds on most PCBs comes from a solder mask. A solder mask however can be found in either blue or red.

Board Outline

The board outline of a PCB can be cut into any shape for a form factor that meets a specific design. When working with small devices, the need for a specific shape (round, rectangular, zig-zag etc…) is important to finalizing a product. As such a number of methods are used to define the shape of your board outline, including importing DXF files (a format used by mechanical CAD tools) to help to define a specific shape for the design.

Creating Copper Routes

Copper routes on a Printed Circuit Board are responsible for conducting the electrical signal throughout the board, to various components and connectors. The copper is created either by layering copper on the completed board surface(s) and etching away excess copper to create the copper pathways. These etchings are created by placing a temporary “mask” over areas of copper routes, and then removing all unwanted copper.

Drilling Holes

In order to create pathways of signals to various layers on a board, or to create regions to attach components on a board, you do need to drill holes on a board. A plated through hole (PTH) in a printed circuit board is called a via and allows you to provide electrical connection between a copper route on one layer of a PCB, to copper on another layer. There are various types of vias. A blind via starts on one outer layer of a PCB, but ends at an inner layer (i.e. it does not completely pass through the board). A buried via connects copper routes on two inner layers of a board (i.e. it does not connect at the surface level of a board). Holes for vias are “drilled” using either a fine drill bit, or in the case of very small microvias with a laser.

Components on a PCB Design

Components on PCB are the semiconductor devices that together allow you to perform specific design actions (filtering, amplification etc…). Components are either through-hole technology (THT) components or surface mount devices (SMD).

THT parts are generally larger in size and were ubiquitous in design until the late 1980’s when SMD parts became more widely used. THT have longer pins that are basically inserted into drilled holes and soldered one-by-one onto the PCB.

SMD parts are (generally) much smaller in size and allow you to solder much smaller leads to the surface of a PCB. By having the SMD parts soldered to the surface of a PCB, engineers can attach parts to either surface (top or bottom) of a PCB, rather than having to solder through-hole parts.

Gerber Files

A Gerber File is a file format used for PCB manufacturing. Fabrication machines can use these files to layout electrical connections such as trace and pads. The file also contains information for drilling, and milling the completed circuit board.